There are two types of sensory receptor cell in the mammalian hearing organ (the cochlea), the so-called inner and outer hair cells. Inner hair cells convey most auditory information to the brain, whereas the role of the outer hair cells may be to amplify the input to the inner hair cells. Our work is designed to describe the operation and the role in hearing of these two cell types. Intracellular recordings are made from these cells in anesthetised guinea pigs. Our techniques allow recordings from both cell types and from different locations along the cochlea. This is important because different sound frequencies are processed at different cochlear locations. There is some evidence that the mode of operation of outer hair observations, however, are based on different techniques and, consequently, are not incontrovertible. We intend to use identical techniques in recording from all cochlear locations and thus to ascertain whether or not there are longitudinal differences in outer hair cell function. Without inner hair cells there is no hearing. Without hearing hair cells, hearing is severely deteriorated. There are over 20 million hearing impaired individuals in the U.S. The majority of them suffer their handicap due to hair cell, particularly outer hair cell, damage. An understanding of the neurobiology of hair cells is a key to prevention and eventual remediation of hearing loss. Our work has been and is designed to provide such fundamental understanding.